Alkali baths for metal treatment



United States Patent Claims priority, application GreatBritain Jan. 30, 1957 6 Claims. (Cl. 148-62) This invention provides an improved process for the treatment in aqueous alkali baths of metallic surfaces.

The invention also provides a concentrate, suitable on dilution, for use in these aqueous alkali baths.

The treatment of metals and alloys in alkali solutions is commonly undertaken to remove grease, paint, enamel, scale, oxide layers and incrustations, etc., and to roughen or etch metals such as aluminium and zinc and their alloys. There is frequently an evolution of fume and spray from baths used for such treatments due to the generation of hydrogen or other gas at the metal surface, or to the liberation of steam. There is also a considerable loss of heat due to evaporation from the surface of these treatment baths when they are operated substantially above room temperature.

The evolution of alkali spray and steam result in wastage of heat and constitute a nuisance to the operators unless exhausting equipment is installed.

In this invention the evolution of alkali spray and steam, from a tank, bath or pot, containing a solution of alkali is largely eliminated by having on the surface of the alkali solution a layer of an inert substance which is liquid at the temperature of use of the alkali bath and substantially immiscible therewith. The loss of heat by surface evaporation of steam is markedly reduced by such a surface layer; this is particularly useful when the bath is being kept hot but not in constant use. Chemical losses, from the bath, are also reduced by the scrubbing action of the inert surface layer.

This invention, therefore, provides a process of treating metal comprising immersing the metal in aqueous alkali having floating on its surface a layer of liquid saturated hydrocarbon material.

In one form of the invention the alkali solution and surface layer contain a surface active agent.

The invention also provides concentrates, suitable on dilution, for use in aqueous alkali baths comprising a saturated hydrocarbon material and a water-soluble alkali. In addition the concentrate may contain a surface active agent.

The layer forming material being saturated, i.e. containing no unsaturated carbon to carbon bonds, is inert, i.e. stable against reaction with the alkali in the bath or with atmospheric oxygen, at the temperature of operation of the bath. It must, of course, also be less dense than the alkali solution, It is preferable that it contains very little or no material volatile at the temperature of operation of the bath and it should be of such viscosity at the operating temperature of the bath that the layer will readily reform after being broken by, for example, the entry or exit of articles to the bath, or the exit of bubbles of gas or vapour.

The preferred layer forming materials have a viscosity of from 0.005 to 0.02 poise and though the lower viscosity materials are preferred care should of course be ".ken that the flash point of the material is above that of the temperature of operation of the process.

The action of the alkali bath may be speeded by application of an electric current, either alternating or direct, or both together, and the additional gas formation would normally cause the evolution of more fume and spray. A surface layer as described in this specification is particularly useful in such cases.

Patented June 6, 1961 There is an advantage in having a surface active agent present in the aqueous bath to facilitate the removal of any adherent surface layer carried down on the metal surface on immersion. In non-quiescent baths, i.e. in baths operated at the boiling temperature of the aqueous alkali or in which there is vigorous effervescence from the metal being treated, it is preferred that the surface tension of the aqueous alkali be not reduced below that of the floating layer. It is an additional advantage to employ a surface active agent which is soluble both in the alkali bath and in the surface layer since the presence of surface active agent in the surface layer facilitates removal of any adherent surface layer on subsequent rinsing. Further the'surface layer can act as a reservoir to take up any excess surface active agent which may throw out of solution in the bath at elevated temperatures.

The surface active agent must obviously be one which is stable in an alkali bath and the amount present must not be suiiicient to emulsify the surface layer into the alkali bath. Non-ionic surface active agents are preferred for use in this invention because they are more stable in the solutions encountered.

The invention is particularly useful when applied to alkali baths used in chemically milling certain metals to produce for example, very thin sheets having high strength to weight ratio. Ribs are produced on these sheets by etching areas of metal, the ribs being formed by protecting chosen areas from attack by the etching solution.

Example I Degreased aluminium panels were deeply etched by immersion for five minutes in 20% sodium hydroxide solution containing 0.1% of a non-ionic surface active agent (an anhydrous condensation product of ethylene oxide and a long chain fatty alcohol) at 40 C. with a 12 mm. layer of a hydrocarbon material having a flash point of C. and a viscosity of 5.1 poises at the temperature of operation, on the surface. There was no noticeable evolution of spray or fume. A filter paper treated with 1% phenol phthalein solution and held one inch above the surface layer showed no alkaline reaction in 5 minutes. The surface active agent was present in this example to facilitate the removal of layer material cartied down on the panels and so promote uniform etching of the metal.

Example II Painted steel panels were immersed in 20% sodium hydroxide solution containing 0.1% of an anionic surface active agent (a secondary alkyl sulphate) at 100 C. for 60 minutes with a 12 mm. layer of hydrocarbon material having a flash point of C. and a viscosity of 5 centipoises at 95 C. on the surface. All paint was successfully removed and there was no noticeable evolution of spray or steam from the solution during the treatment.

Example III A solution of the following compositions was prepared:

Percent by weight Sodium orthosilicate 2.9 Non-ionic surface active agent (long chain fatty alcohol condensate with ethylene oxide) 0.1

and covered with 12 mm. layer of hydrocarbon material as used in Example 11. The solution was then heated to 90 C. and greasy steel panels were passed through the surface layer into the solution. After 5 minutes treatment the panels were withdrawn through the surface layer and rinsed by immersion in cold running water. The panels were fully wetted by the water and it was judged that no trace of oil or grease remained.

was covered with a 12 mm. layer of hydrocarbon material as used in Example II and heated to boiling. Aluminum panels were treated in this bath for 15 minutes followed by rinsing and drying. These panels showed greater corrosion resistance than untreated metal when subjected to salt spray test.

Concentrates suitable on dilution for making up the baths used in the improved process of treating metal comprise a mixture of the liquid saturated hydrocarbon material and water-soluble alkali or concentrated alkali solution. Surface active agent, if required in the process, may also be incorporated in the concentrate. When the inert liquid is mixed with alkali powder in proportions suitable for eventual use the product usually retains its powdery characteristics and the inert liquid in many cases prevents caking of the alkali. In all cases the concentrate is a convenient way of preparing, handling and storing, the ingredients in a form which can readily be put to use.

Since the inert layer on the alkali baths is very effective in reducing steam and fume evolution from the baths it is desirable to colour the layer to warn operators that the bath under the layer may be very hot although the usual visible signs are not present. The colour may be introduced into the layer by dissolving in it a red dye.

Hydrocarbon material (as used in Example H) 80 Waxoline red dye 0.01

91 lbs. of this composition were added to a bath 'containing 100 gallons of water and having an upper surface of eight square feet in area. The salts were dissolved and the hydrocarbon material formed a layer 16 mm. thick and coloured red on the upper surface of the solution. The temperature of the solution was raised to 98 C. and aluminum panels immersed in the hot solution for 15 minutes received an excellent corrosion resistant coating.

What I claim is:

1. In a process involving alkali treatment of a metal wherein the metal is immersed in an aqueous alkali solution with the evolution of undesired fumes or spray, the improvement whereby said evolution of fumes and spray is minimized, said improvement comprising immersing said metal in an aqueous alkali solution covered with a saturated hydrocarbon surface layer which is liquid at the operating temperature, said liquid hydrocarbon being substantially immiscible withrsaid alkali solution, subr 4 stantially non-volatile and stable at the operating temperature of said solution and inert with respect to the constituents of the solution, both said solution and said surface layer having a surface active agent dissolved therein.

2. A process as claimed in claim 1 in which the aqueous alkali solution is boiling or vigorously effervescent and contains the surface active agent in amount insufficient to reduce its surface tension below that of the liquid hydrocarbon material.

3. A process as claimed in claim 1 in which the surface active agent is a long chain fatty alcohol condensate with ethylene oxide. 1

4. A process of treating metal as claimed in claim 1 in which the aqueous solution contains chromate ions.

5. In a process involving alkali treatment of a metal wherein the metal is immersed in an aqueous alkali solution with the evolution of undesired fumes or spray, the improvement whereby said evolution of fumes and spray is minimized, said improvement comprising initially covering the surface of a heated alkali solution, prior to immersing any metal therein, with a layer of saturated hydrocarbon which is liquid at the operating temperature, substantially immiscible with said solution and has a viscosity of from 0.005 to 0.02 poise, said liquid being stable and substantially non-volatile at the operating temperature of said solution and inert with respect to the constituents of said solution, including in both the surface layer and said solution a surface active agent, which is soluble in both said layer and solution, said surface active agent having sufficient solubility in said surface layer to take up any surface active agent separated out from said solution, and only thereafter immersing the metal through said layer and into said solution to effect said alkali treatment.

6. An aqueous alkali metal treating bath consisting essentially of an aqueous alkali solution covered with a non-aqueous surface layer of a saturated hydrocarbon which is liquid at operating temperature, said liqliitfhydrocarbon being substantially immiscible with said solution, substantially non-volatile and stable at the operating temperature of said bath and inert with respect to the constituents thereof, both said solution and said layer having a surface active agent dissolved therein.

References Cited in the file of this patent UNITED STATES PATENTS 585,355 Burghardt et al June 29, 1897 1,838,633 Pacz Dec. 29, 1931 2,227,945 Neilson Ian. 7, 1941 2,247,580 Jauch July 1, 1941 2,250,473 De Long July 29, 1941 2,493,327 Vance Ian. 3, 1950 2,796,361 Chester et a1 June 18, 1957 2,837,449 Blaser June 3, 1958 FOREIGN PATENTS 270,514 Great Britain May 12, 1927 856,526 France Mar. 23, 1940 

1. IN A PROCESS INVOLVING ALKALI TREATMENT OF A METAL WHEREIN THE METAL IS IMMERSED IN AN AQUEOUS ALKALI SOLUTION WITH THE EVOLUTION OF UNDESIRED FUMES OR SPRAY, THE IMPROVEMENT WHEREBY SAID EVOLUTION OF FUMES AND SPRAY IS MINIMIZED, SAID IMPROVEMENT COMPRISING IMMERSING SAID METAL IN AN AQUEOUS ALKALI SOLUTION COVERED WITH A SATURATED HYDROCARBON SURFACE LAYER WHICH IS LIQUID AT THE OPERATING TEMPERATURE, SAID LIQUID HYDROCARBON BEING SUBSTANTIALLY IMMISCIBLE WITH SAID ALKALI SOLUTION, SUBSTANTIALLY NON-VOLATILE AND STABLE AT THE OPERATING TEMPERATURE OF SAID SOLUTION AND INERT WITH RESPECT TO THE CONSTITUENTS OF THE SOLUTION, BOTH SAID SOLUTION AND SAID SURFACE LAYER HAVING A SURFACE ACTIVE AGENT DISSOLVED THEREIN. 